I don't know why that may be unless the SS mics are like CSB's. The CSB's don't work too well line in since they aren't sensitive mics, but they do get better levels whilst ran mic in.
If you don't want to run mic-in on the HiMD, my suggestion would be to get a pre for you SS mics, like and AD20, I ran my old CSB's into for a while with good results.